Aidan wrote:

Agree, it's a 4 gallon feed and expansion tank for the heating. If it
has distorted, maybe it's had very hot water in it when the heating was
running; this would indicate a fault. It should always be cold.

You probably need to replace the washer in the float valve/ballcock. It
should have about 1" or 2" of water covering the outlet when the
heating system is cold, leaving about 3 gallons of airspace to
accomodate the expansion of the water when the system is hot. They are
very frequently set up incorrectly, with 3 or 4 gallons of water when
cold. The water will then overflow when the system heats up, and will
take in cold water when it cools down. This will ruin the heating
system in a few years.

What can also happen if the vent pipe (upside down U shaped pipe with open
end over the tank) is too close to the water level is that water can
continously pump over from the vent pipe into the header tank which then
gets heated up to the temperature of water from the boiler. The plastic
float valve may then burst and let in water as the system cools down, so it
no longer floats and so the float valve opens and lets water in to the tank
until it flows out of the overflow: this also raises the water level in the
tank encouraging pumping over to continue.

Sometimes the vent pipe is supposed to be fixed to something and has come
loose. It should stop just below the rim of the tank, so that it's well
above the water level. It should also go up 6" or so above the level of the
open end.

As Aidan says the water level in the tank should be quite low when the
system is cold, maybe 3" (it will rise when the system heats up and the
water expands). To maintain this level the brass rod on the float valve
will usually have to be bent down.